Hydrophilic interaction liquid chromatography-electrospray ionization-tandem mass spectrometry of a complex mixture of native and oxidized phospholipids

A mixture of native and oxidized phospholipids (PLs), generated by the soybean lipoxygenase type V-catalyzed partial oxidation of a lipid extract obtained from human platelets, was analyzed by Hydrophilic Interaction Liquid Chromatography-ElectroSpray Ionization-Tandem Mass Spectrometry (HILIC-ESI-MS/MS). The complexity of the resulting mixture was remarkable, considering that the starting lipid extract, containing (as demonstrated in a previous study) about 130 native PLs, was enriched with enzymatically generated hydroperoxylated derivatives and chemically generated hydroxylated forms of PLs bearing polyunsaturated side chains. Nonetheless, the described analytical approach proved to be very powerful; indeed, focusing on phosphatidylcolines (PCs), the most abundant PL class in human platelets, about fifty different native/oxidized species could be identified in a single HILIC-ESI-MS/MS run. Low-energy collision induced dissociation tandem MS (CID-MS/MS) experiments on chromatographically separated species showed single neutral losses of H2O2 and H2O to be typical fragmentation pathways of hydroperoxylated PCs, whereas a single H2O loss was observed for hydroxylated ones. Moreover, diagnostic losses of n-hexanal or n-pentanol were exploited to recognize PCs hydroperoxylated on the last but five carbon atom of a ɷ-6 polyunsaturated side chain. Despite the low resolution of the 3D ion trap mass analyzer used, the described HILIC-ESI-MS/MS approach appears very promising for the identification of oxidized lipids in oxidatively stressed complex biological systems.

Phospholipidomics of human blood microparticles

The phospholipidome of blood microparticles (MPs) obtained from platelet-rich plasma of healthy individuals was characterized by hydrophilic interaction liquid chromatography (HILIC) coupled to electrospray ionization tandem mass spectrometry (ESI-MS/MS). The HILIC separation, performed on a silica stationary phase using an acetonitrile/methanol gradient, enabled the separation of several phospholipids (PL) classes, viz., phosphatidyl-cholines (PCs), -ethanolamines (PEs), -serines (PSs), -inositoles (PIs), sphyngomielins (SMs), and lyso forms of PCs and PEs. Structural characterization of species belonging to each class was performed by MS/MS measurements, in either positive or negative ion mode. The set of 131 phospholipids (including regioisomers) here identified represents the most comprehensive phospholipidomic characterization reported for human MPs. Although the phospholipidome composition of MPs and platelets, collected from the same donors, was found to be qualitatively the same, quantitative differences were evidenced for lyso-PCs, which appear to be significantly more abundant in MPs.

Improved specificity of cardiolipin peroxidation by soybean lipoxygenase: a liquid chromatography - electrospray ionization mass spectrometry investigation

Peroxidation catalysed by Soybean Lypoxigenase was performed on tetralinoleyl-cardiolipin with the aim of generating selectively oxidized products, to be used subsequently as standards for studies on cardiolipin oxidation. The reaction products were characterized by LC-ESI-MS and MS/MS, and the process was found to link a hydroperoxylic group on one or more linoleic chains of cardiolipin, up to a total of four groups per molecule. Interestingly, the incidence of other oxidized products, like those arising from multiple hydroxylation or mixed hydroxylation-hydroperoxydation, previously observed after the chemical oxidation of the same cardiolipin, was found to be negligible. Moreover, evidences for the presence of the hydroperoxylic group(s) almost exclusively on carbon 13 of the linoleic chain(s) were obtained by MS/MS measurements. The enzymatic approach, integrated with a preparative separation step, which could be developed by adapting the chromatographic conditions adopted in the present work for analytical purposes, represents a promising strategy for the synthesis of highly specific mono- or multi-peroxidated derivatives of cardiolipins.

Annexin V binding perturbs the cardiolipin fluidity gradient in isolated mitochondria. Can it affect mitochondrial function?

The phosholipid bilayer fluidity of isolated mitochondria and phospholipid vesicles after calcium-dependent binding of annexin V was studied using EPR spectroscopy. The membranes were probed at different depths by alternatively using cardiolipin, phosphatidylcholine, or phosphatidylethanolamine spin labeled at position C-5 or C-12 or C-16 of the beta acyl chain. Computer-aided spectral titration facilitated observing and quantitating the EPR spectrum from phospholipid spin labels affected by annexin binding, and spectral mobility was calibrated by comparison with standard spectra scanned at various temperatures. In most cases it was found that binding of the protein to the membranes makes the inner bilayer more rigid up to acyl position C-12 than afterward, in agreement with the previously observed effect in SUVs [Megli, F. M., Selvaggi, M., Liemann, S., Quagliariello, E., and Huber, R. (1998) Biochemistry 37, 10540-10546]. Moreover, in isolated mitochondrial membranes, cardiolipin apparently is more readily affected than the other main phospholipids, while in vesicles made from mitochondrial phospholipids, the different species are affected in essentially the same way. This behavior is consistent with the existence of distinct cardiolipin pools in mitochondria, and with the already advanced hypothesis that these domains are the binding site for annexin V to the isolated organelles [Megli, F. M., Selvaggi, M., De Lisi, A., and Quagliariello, E. (1995) Biochim. Biophys. Acta 1236, 273-278]. Keeping in mind the funcional importance of cardiolipin in the mitochondrial membrane, the question is raised as to whether the observed influence of annexin V binding to this phospholipid and its consequent local fluidity alteration might affect the mitochondrial functionality, at least in vitro.

The calcium-dependent binding of annexin V to phospholipid vesicles influences the bilayer inner fluidity gradient

The fluidity of the hydrophobic interior of phospholipid vesicles after calcium-dependent binding of human annexin V (AVH) was studied using EPR spectroscopy. Vesicles (SUVs) composed of PC or PE and an acidic phospholipid (alternatively PS, PA, or CL) were probed at different bilayer depths by either phosphatidylcholine, or the accompanying acidic phospholipid, bearing a spin label probe at position C-5, C-12, or C-16 of the sn-2 acyl chain. Alternatively, the vesicle surface was probed with a polar head spin labeled PE (PESL). The EPR spectra of annexin-bound bilayer domain(s) were obtained by computer spectral subtraction. The order parameter values (S) from the resulting difference spectra revealed that the bilayer hydrophobic interior has a greatly altered fluidity gradient, with an increased rigidity up to the C-12 position. Thereafter, the rigidification progressively vanished. The effect is not linked to the phospholipid class, since all the acidic phospholipid spectra, as well as phosphatidylcholine, shared the same sensitivity to the bound protein. The observed membrane rigidification appears to parallel the "crystallizing" tendency of vesicle-bound annexin V, but may not be involved in the calcium channeling activity of this protein.

EPR study of annexin V-cardiolipin Ca-mediated interaction in phospholipid vesicles and isolated mitochondria

The properties of the binding of annexin V to variously composed phospholipid vesicles have been studied by applying a recently developed EPR method, using an annexin V spin label. By this approach, this protein is seen to bind to acidic phospholipid-containing vesicles, as reported, thus confirming the reliability of the method. In addition, binding of this annexin to cardiolipin-containing vesicles has been studied in more depth, and the protein has been shown to have a distinct affinity for this phospholipid. As a cardiolipin-rich natural membrane system, mitochondrial membranes and mitoplasts from rat liver were considered, and a strong binding of AV to these membranes was observed. Having compared this binding with that to phospholipid vesicles, cardiolipin-rich microdomains in the mitochondrial membranes are proposed as the putative mitochondrial binding sites for annexin V.

An EPR method for studying annexin-biomembrane interaction

Annexin p34 spin label has been used as a probe for studying annexin-membrane Ca-mediated interaction. The EPR spectrum of the purified probe is able to monitor the binding of the protein to phospholipid vesicles and isolated mitochondrial membranes, with increasing calcium concentration, as revealed by the calculated rotational correlation time. A novel mathematical approach is proposed to provide a direct estimation of annexin percentage binding to membranes, based on the correlation time calculated from experimental EPR spectra.

Spin labeling of calcium-dependent phospholipid-binding proteins

Bovine lung annexins p32 and p34 were spin labeled with an iodoacetamidoproxyl spin label, a reagent that reportedly couples with protein methionine residues. Labeling conditions and stoichiometry were studied with the radiolabeled analogue [1-14C]iodoacetamide. As judged by this method, carboxamidomethylation of both p32 and p34 occurred up to a 0.7 mol ratio after 60 h of reaction at 37 degrees C and at pH 4. The two proteins retained Ca2(+)-dependent phospholipid-binding ability both in radiolabeled and in spin-labeled forms. Electron resonance spectra of spin-labeled p32 and p34 showed the features of a partially immobilized spin probe, with rotational correlation time values of 1.15 and 1.25 ns, respectively, which definitely indicate successful spin labeling. Quantitation of ESR spectra by computer double integration indicated 70% spin labeling of both proteins, as anticipated by radiolabeling. The use of spin-labeled p32 and p34 in the study of Ca2(+)-dependent interaction of annexins with biomembranes is proposed.

Nonspecific lipid transfer protein (sterol carrier protein 2) is bound to rat liver mitochondria: its role in spontaneous intermembrane phospholipid transfer

In the present study we have investigated the transfer of phospholipids between vesicles and rat liver mitochondria. Transfer was measured by electron paramagnetic resonance spectroscopy using vesicles that contained spin-labeled phospholipids. A spontaneous transfer was observed which could be strongly inhibited by treating the mitochondria with the thiol reagent mersalyl. Transfer was also greatly reduced after a saline wash of the mitochondria; the transfer activity was then recovered in the wash. This activity was inhibited by tryptic digestion and mersalyl. By gel chromatography, enzyme immunoassay and immunoblotting it was demonstrated that the activity in the wash was due to the nonspecific lipid transfer protein (sterol carrier protein 2). We could estimate that up to 85% of the spontaneous phospholipid transfer between vesicles and rat liver mitochondria was mediated by this transfer protein.

Chemical modification of methionine residues of the phosphatidylcholine transfer protein from bovine liver. A spin-label study

The role of methionine residues in the interaction of the phosphatidylcholine transfer protein from bovine liver with phospholipid vesicles was investigated by specific modification of these residues with iodoacetamide. The modified protein was digested with cyanogen bromide in order to determine which methionine residues had become resistant to this cleavage. Automated Edman degradation on the digest indicated that after 72 h of reaction, Met-1 was modified for 80%, Met-73 for 50%, Met-109 for 20%, whilst Met-173 and Met-203 were found to be unmodified. This distinct modification did not result in any loss of phosphatidylcholine transfer activity. The interaction of the phosphatidylcholine transfer protein with phospholipid vesicles was investigated by making use of electron spin resonance spectroscopy. The interaction of unmodified protein with vesicles composed of phosphatidylcholine/phosphatidic acid/spin-labeled phosphatidylethanolamine (79:16:5, mol%) or composed of phosphatidylserine/spin-labeled phosphatidylethanolamine (95:5, mol%), gave an increase of about 50% in the rotation correlation time. A similar increase was observed with the modified protein. This interaction was further investigated by labeling Met-1 and Met-73 in the transfer protein with iodoacetamidoproxyl spin-label. Spin-labeling did not inactivate the transfer protein. In addition, the electron spin resonance spectra of the spin-labeled protein were not affected upon addition of vesicles composed of phosphatidylcholine/phosphatidic acid (80:20, mol%). These experiments strongly suggest that Met-1 and Met-73 are not part of the site that interacts with the membrane.

Characterization of sulphydryl groups of the mitochondrial phosphate translocator by a maleimide spin label

A maleimide spin label strongly inhibits the phosphate/H+ symporter of rat liver mitochondria. While inducing half-maximal inhibition of transport, the spin label reacts preferentially with the SH groups of the carrier, which are at least of two types. One type of SH group is localized close to the surface of the membrane and its environment does not significantly influence the mobility of the probe. The second type of SH group is buried in the membrane, is not accessible to ascorbate or chromium oxalate and its environment greatly restricts the motion of the probe.

[Comparative study of Lecithin and cephalin exchange between liposomes and mitochondria. Activity measured in the presence and absence of a non-specific carrier protein isolated from rat liver]

Study has been made of the rat liver protein catalyzed exchange of both lecithin and cephalin between liposomes and rat liver mitochondria. It has been shown that the exchange activity of these two phospholipids by the protein is almost the same and is apparently not dependent on the nature of donor liposomes. In contrast the spontaneous exchange activity of the above phospholipids strictly depends on the type of donor liposomes. Moreover, the spontaneous exchange of lecithin at any incubation time appears to be almost 100% higher than that of cephalin.

An in vitro ESR study of uncatalyzed rat liver protein-catalyzed spin-labeled phosphatidylcholine exchange

ESR spectrometry has been used to study fatty acid spin-labeled phosphatidylcholine exchange from single bilayer donor vesicles to various acceptor systems, such as intact or differently treated mitochondria, phospholipid multilamellar vesicles or single bilayer vesicles. This exchange is catalyzed by soluble non-specific rat liver protein, first investigated by Bloj and Zilversmit in 1977 (J. Biol. Chem. 252, 1613--1619). Non-catalyzed phosphatidylcholine exchange has also been studied. Full inhibition of both mechanisms occurs with lipid-depleted acceptor mitochondria, while N-ethylmaleimide-treated mitochondria behave as good acceptors during catalyzed exchange but are in no way effective during spontaneous exchange. Non-catalyzed exchange does not take place with phospholipase D-treated mitochondria as acceptors, while the pure catalyzed mechanism is inhibited by 28%. Neither multilamellar nor single bilayer phospholipid vesicles exchange spin-labeled phosphatidylcholine in the absence of protein, the former being a poorer acceptor system than the latter during catalyzed exchange, when this activity is 31 and 80%, respectively, of that of intact mitochondria. The hypothesis is made that the spontaneous mechanism is active among intact natural membranes and could be of some importance in vivo. Furthermore, the biomembrane protein moiety is assumed to be involved in the catalyzed exchange more as a phospholipid spacer than as a binder between the exchange protein and the membrane involved. Phospholipids, on the contrary, appear to be important for both functions.

[Characteristics of a rat liver cellular extract protein which mediates transport of various phospholipids from liposomes to mitochondria]

The effect of pH, Ca++ and Na+ concentration on the phosphatidylethanolamine transport from liposomes to mitochondria by an aspecific soluble rat liver protein has been studied. Results obtained indicate that at pH higher than 6.5 and Ca++ ion concentrations from 5 mM on the transport is strongly inhibited. Preliminary data with ESR spectrometry concerning the nature of lipoprotein association between phosphatidylethanolamine and carrier protein during the transport are also presented.

Turnover of fatty acids in rat liver cardiolipin: comparison with other mitochondrial phospholipids

Following intraperitoneal administration of 1-14C-linoleic acid or 2-3H-acetate to rats, the specific radioactivities of both liver cardiolipin and other mitochondrial phospholipids after different time intervals were measured. Comparison of the data obtained with those from another stock of rats treated with 32P-phosphate or 2-3H-glycerol showed that the fatty acids of cardiolipin, like those of other phospholipids, exhibit an independent turnover with respect to the remaining parts of the molecule. The half-life of acyl moieties of cardiolipin is ca. 20% higher than that of the same components of other mitochondrial phospholipids. Moreover, it appears that, in both cardiolipin and other phospholipids, linoleyl residues turn over faster than nonessential fatty acids. Discussion is made as to whether this characteristic can be related to the role of phospholipids in the functioning of some enzymes bound to the inner mitochondrial membrane.